1. Field of the Invention
The invention relates to the control of electric actuators and, more particularly, to the operation of electric motors under programmed current mode control.
2. History of the Prior Art
The basic laws of physics governing the operation and control of electric motors are well known in the prior art. For example, it is a fundamental principle that the flow of an electric current through a magnetic field produces a force. Thus, the relationship which describes the force and/or torque produced by an electric motor may be expressed in general terms as: EQU T=KI;
where T is torque, K is a constant determined by the construction of the motor; and I is the current through the motor.
Although this principle is well known, and, moreover, forms the basis for all electric motor design, the techniques for operating electro-dynamic control systems have all followed the same general path. For example, a typical feedback controlled electric motor/actuator includes a variable voltage or current source for driving the motor, a selected performance reference to be achieved (such as position, velocity or acceleration of the motor shaft), and some means to measure the actual performance of the motor (such as position, velocity or acceleration of the motor shaft). The reference value and the measured performance are compared and an error signal is generated based on the difference between them. The error signal is then used to change the voltage or current driving the motor in a direction and value so as to change the motor torque and tend to make the measured parameter performance and the reference value equal and the error signal zero.
Instability is inherent in all conventional feedback control systems. Such systems necessarily include one or more external transducers monitoring the performance parameters of the system. These transducers are almost always analog devices, the output of which is compared with a reference to produce an analog error signal. This error signal is amplified and used to control the motor and drive the error signal toward zero. In all closed loop feedback systems the gain and phase of the error signal must be kept within a specific relationship to the driving signal, to prevent instability and oscillation of the system. The nature of such closed loop feedback systems is that the feedback signal is never in precise synchronism with the driving signal, and further, the degree of lead or lag in the feedback signal is a function of the transfer function of the link between the driving motor and the performance elements being monitored by transducers. All of these consideratios, inherent in conventional closed loop feedback control systems, contribute to potential instability of the system and serve to severely restrict the response time within which such systems can operate.
The relationship between the value of the current flowing in a magnetic field and the value of the force produced thereby has been used for many years to determine either an unknown force or an unknown current. For example, U.S. Pat. No. 3,968,850 to Gaskill and U.S. Pat. No. 4.365,680 to Gottstein et al both disclose weighing systems which measure the current required to generate a force sufficient for balancing the weight of an object. They also include means for damping oscillations to rapidly stabilize the system when a new measurement is being made.
Present current measurement force detection systems conventionally require the accurate sensing of relatively low level analog signals. In an industrial environment, where there is almost always a large component of electrical noise, the precise sensing of such low level signal values is very difficult to achieve in the best of circumstances and impossible to achieve in the worst of circumstances. For example, U.S. Pat. No. 4,564,910 discloses a technique for minimizing error due to noise generated signal fluctuations from a torque transducer, by mechanically smoothing the force function into a series of force signal increments. However, this technique does not address the fundamental problem of accurately measuring low level analog current values in such a high electrical noise level industrial environment.
The system of the present invention adopts a fundamentally different approach to both the conventional measurement of output force parameters with performance transducers, as well as the traditional implementation techniques of closed loop feedback control servomechanism, to effect superior force determinations and control in electrically acutated apparatus. The system of the present technique not only increases the stability of electrical acuator controlled systems but dramatically increases the response time (i.e. speed) within which such systems can operate.